Reading Directive/2004/38/EC's blog, I found the following:
This paragraph is written with non-EU family members in mind but I see no reason why it wouldn't apply to the EU citizen itself. My partner and I (both EU citizens; this is to get PR in order to apply for naturalisation) went to the UK in February for interviews, returned in July to look for flats and then moved to the UK properly in September (not this year but all in the same year).If you came for a short visit to the host state with your EU family member, e.g. for house hunting or for job interviews, and then went back to your previous residence before fully moving to the host state, then your residence most likely began with the short visit. For this to be valid, the EU citizen would have to begin exercising their treaty rights within 3 months of the short visit (or longer if they were actively looking for work).
Previously I assumed my 5 year period for PR started in September but based on this blog it seems I could make the claim that it started in July (assuming one of us started exercising treaty rights in September). Is this interpretation correct?